This invention relates to an improvement in a diagnostic X-ray apparatus utilizing a digital X-ray subtraction imaging technique.
The digital X-ray subtraction imaging technique using an imaging intensifier-TV chain was developed and first described by Mistretta et al. For example, U.S. Pat. Nos. 4,204,225 and 4,204,226 issued to Charles A. Mistretta disclose real-time digital X-ray subtraction imaging methods and apparatus which are used in the so-called digital radiography or digital fluoroscopy technology.
These digital X-ray subtraction imaging apparatus include an image intensifier, a TV camera whose output is converted into digital format, and an image processor incorporating two digital memories. As disclosed in U.S. Pat. No. 4,204,225, in carrying out the mask mode of this technology a first memory is preferably employed to integrate and store digital mask video signals by digitization of video signals from an image intensifier-TV chain during an initial mask time interval utilizing a relatively large number of television fields. A second memory system integrates ongoing digital video signals and provides an output of these integrated video signals from which the mask video signals from the first memory system are subtracted. The resulting digital difference video signals are supplied to a digital-to-analog converter which provides corresponding analog difference video signals for display by a display device. Alternatively, the analog signals may be fed to a multiformat camera for making a selected number of radiographic exposures on a single film.
In the time interval difference mode disclosed in U.S. Pat. No. 4,204,226, a series of difference images is produced by integrating digital video signals over a series of successive time intervals, performing a series of subtractions between the sets of successive integrated video signals stored in the memories to produce a series of digital difference video signals, and converting such digital difference video signals into visibly displayed difference images representing changes in the X-ray image during the successive time intervals.
One advantage of digital radiography apparatus is its capability to perform angiography by means of intravenous injections instead of by intra-arterial catheter techniques with their higher risks. Still another advantage of digital radiography apparatus is its capability to provide improved low contrast detectability, namely, to amplify subtle amounts of contrast media in arteries better than film methods can. Another advantage of digital radiography apparatus employing pulsed X-rays is that significant loss of spatial resolution due to physiological motion can be prevented because the short radiation pulse for each image results in less loss of detail during fluoroscopy of moving objects.
In these digital radiography apparatus, the digital X-ray subtraction imaging technique is used to remove or at least reduce potentially confusing effects of uninteresting overlying and underlying objects and thereby to enhance the detectability of the structures of interest. In this process images are first produced without and then with contrast media present and unwanted images are eliminated since all structures common to both images are cancelled by subtracting the first image from the second. However, some unsubtracted images due to residual contrast media frequently accumulating in uninteresting and unwanted vascular areas, as to the analysis, appear in the result images because the subtraction leaves changes in the contrast medium distribution arising with time differences. These unsubtracted images reduce image quality and obscure the detectability of the structures of interest.